This invention relates generally to method and apparatus for monitoring the weight load on a vehicle such as a truck, trailer or the like in which at least one strain gage sensor is internally mounted within a shackle pin that couples a trailer suspension bracket to an axle-mounted leaf spring for on-the-road, real time monitoring of dynamic as well as static load conditions.
Methods for weighing payloads are common on many types of vehicles, such as trucks, vans and other heavy payload vehicles, where weight distribution is an important factor. Operators of trucks driven over interstate highways must monitor the weight of the truck payload for several reasons. Rig operators must remain in compliance with the legal load limits to avoid paying substantial fines levied for weight violations. Also, a driver does not want to carry an excessive load that may damage the trailer, overload the tires and suspension, cause excessive wear on the engine, cause premature wear on the brakes, and reduce fuel efficiency. Additionally, a driver needs to know immediately if the payload has shifted so that he may take emergency measures to restore balance and secure the load before the trailer becomes unstable or unsafe.
Monitoring the payload carried by a tractor trailer can be a difficult task. A payload is often loaded at a remote site such as a gravel pit or logging operation, or other location where truck scales are not readily accessible.
Various devices have previously been used to measure and monitor the weight of a payload. For example, U.S. Pat. No. 5,811,738 (Boyovich et al.) entitled xe2x80x9cTrunnion-Mounted Weight Measurement Apparatusxe2x80x9d discloses a weight measurement apparatus for determining the weight of a load placed on a wheeled vehicle. The apparatus includes a shackle pin containing strain gages for measuring the stress caused by the load and is connected to the truck by replacing the truck""s conventional trunnion coupling with a trunnion member containing the shackle pin and internally mounted strain gages.
U.S. Pat. No. 3,695,096 (Kutsay), entitled xe2x80x9cStrain Detecting Cell,xe2x80x9d discloses a strain load cell combined with a coupling pin or bolt. The body of the coupling pin is intersected by a longitudinal bore and two pairs of strain gages connected in a bridge circuit are mechanically attached to the internal bore sidewall surface. The longitudinal bore also serves as a lubrication passage through which lubrication is supplied to the trunnion bearings. The strain gage signals are routed through an electrical cable to an external processing unit that includes a signal conditioner that amplifies the low level signals and attenuates high frequency noise.
The Kutsay strain detecting cell illustrates certain performance limitations of conventional load measuring systems. The signal processing unit is remotely located from the load pin. Since the strain gage bridge circuit is carefully balanced for outputting a low voltage signal, the impedance of the connecting cable should be adjusted to provide an impedance match with the input of the signal conditioner. Consequently, calibrated cabling or a wireless transmitter/receiver system is required for connecting the strain gage sensor signal to the processor unit. Also, the lubricant present in the passage will contaminate the strain gage components, attacking the adhesive that bonds the sensors to the load pin sidewall. Such interference has been determined to be the cause of improper sensor attachment, producing irregular, distorted output signals.
Other weight measurement devices for transport vehicles are shown in U.S. Pat. No. 3,754,610 (Paelian et al.), entitled xe2x80x9cLoad Cellxe2x80x9d; U.S. Pat. No. 4,102,031 (Reichow et al.), entitled xe2x80x9cMethod of Installing a Transducer on a Structural Memberxe2x80x9d; U.S. Pat. No. 5,402,689 (Grogan) entitled xe2x80x9cNon-Thread Load Sensing Probexe2x80x9d; and U.S. Pat. No. 5,880,409 (Hartman) entitled xe2x80x9cOnboard Weighing System for Truck Having Single Point Suspension.xe2x80x9d
The data signals generated by a basic measuring device such as a strain gage cell or bridge circuit generally require processing or conditioning before being finally presented to the operator as a load indication. In installations on large vehicles such tractor trailer rigs, the load pin and load sensor circuit are remotely located from the signal conditioning amplifier and data display unit, which are typically installed in the operator cab for on-the-road, real time monitoring of dynamic as well as static load conditions.
Calibrated connecting cables conduct such information to the signal conditioning amplifier, which is usually located in the cab. The cable wiring, which may extend for several feet between the measuring bridge and the conditioning circuit, is subject to inductive pick-up of electromagnetic interference noise generated from various sources that tend to distort the low-level signal output from the measuring bridge, which is typically in the millivolt range. Moreover, the long length of cable wiring introduces unwanted impedances between the bridge circuit and the conditioning amplifier that can degrade the response time and transient overload recovery time of the indicating system.
Conventional strain gage load measuring systems have attempted to overcome these limitations by using a shielded, calibrated cable having a predetermined length and known impedance that is matched with the impedance of the measuring bridge and the conditioning amplifier. However, the calibrated cable is exposed to thermal cycling that causes impedance variations that affect the output of the measuring bridge. Since the cable is calibrated, it is not field-repairable; consequently, a damaged cable must be replaced by a new cable of the appropriate length that has been calibrated to match the particular load sensing circuit and signal conditioner installation on the damaged rig. After cable replacement, the overall system must be audited for accuracy and reliability. Consequently, there is considerable interest in improving such load measuring systems so that rig down-time and maintenance expenses can be reduced, while providing more accurate and reliable load measurements to the rig operator.
The present invention provides improved weight measurement method and apparatus for sensing the weight load on a transport vehicle such as a tractor trailer rig. A load sensing transducer circuit is mounted internally of the load pin or shackle pin and senses the weight load imposed by a trailer and its payload on the leaf springs of the transport vehicle. The load signal produced by the transducer circuit is fed directly into the input of a signal conditioning processor that is also mounted internally of the shackle pin. The conditioned load signal is then transmitted via conventional non-calibrated low voltage signal cabling to a remote data display unit that can be monitored by the truck operator.
The body of the shackle pin is intersected by a longitudinal bore in which multiple strain gage sensors are mounted. A miniature signal processing unit includes a signal conditioner that is totally enclosed within the longitudinal bore and shielded by the metallic body of the surrounding load pin. The strain gage sensors are mechanically bonded by adhesive deposits to the internal bore sidewall of the load pin and are electrically coupled together and to the signal conditioner by internal wiring that is totally shielded by the metallic body of the load pin.
The strain gages develop a signal proportional to the weight of the trailer load for input to the signal conditioner. The load forces imposed on the shackle pin are amplified and filtered by the internal signal conditioner and are sent to the data display unit to calculate the total load borne by each wheel or axle. Preferably, the signal conditioner sends this data via conventional, non-calibrated low voltage signal cabling to a controller and display unit installed in the cab of the transport vehicle for real time monitoring by the rig operator.
The load pin is intersected by a radially offset, longitudinal bore forming an internal lubrication passageway that is isolated with respect to the longitudinal bore in which the strain gage sensors and signal processing unit are mounted. The internal lubrication passageway provides lubrication to a set of bearings located in a suspension bracket that couples the shackle pin and a leaf spring assembly to the trailer frame.
The radially offset lubrication passage is isolated from the longitudinal bore and the electronic signal conditioning components within the bore. Since the strain gages are bonded onto the internal bore sidewall of the pin by adhesive deposits, those deposits are vulnerable to attack by hydrocarbon compounds present in conventional lubrication grease. The offset lubricant passage provides a means for lubricating the bearings while preventing contact of the lubricant with the strain gages, internal wiring and signal conditioner components housed within the main longitudinal bore.
Since the lubrication passage is isolated from the main longitudinal bore passage, the lubrication passage can be pressurized with lubricant without pressurizing the electronic components within the main bore. Otherwise, the signal conditioner, strain gages and internal wiring would be exposed to high impulse lubrication pressure surges that could damage the components or possibly cause a discontinuity in the internal strain gage wiring as a result of bearing lubrication service operations performed during normal periodic maintenance of the vehicle.
The signal conditioner is internally mounted in the load pin and is closely coupled to the internally mounted strain gage sensing circuit by short, internal wiring conductors. Thus there is no need for a calibrated cable or radio transmission device to send the conditioned load sensor signals to the remote data display unit. Conventional, non-calibrated low voltage data transmission conductors are used to connect the conditioned output signals to the remote data display unit. Since there is no requirement for calibration or impedance matching, a damaged signal cable can be quickly repaired or replaced in the field with conventional low voltage signal cabling without significant rig down-time. Morever, different tractors can be attached to the trailer and operated with the installed load sensors and existing display equipment without calibration.
Moreover, because impedance matching is not a limiting factor in the present load pin installation, signal delay and distortion are eliminated, thus overcoming a major limitation of conventional strain gage measuring systems that use calibrated cables. Because the load signals are pre-conditioned at the load pin, signal distortion, noise and impedance problems are avoided. The load monitoring system of the present invention responds immediately and accurately to transient load conditions. Therefore there is no lag time or load signal distortion experienced when measuring and indicating the weight of the load. Thus, it is possible to reliably sense gradual as well as rapid shifting overload conditions as they develop, thus providing an early warning of an impending dangerous load condition, allowing the operator to stop the transport vehicle and balance the load or take other corrective action at the onset of a load problem, before the trailer becomes unstable.